Fixed Bit-Rate Optical Access Networks
Optical access networks, such as point to multipoint optical access networks are known in the art. ITU-T Recommendation G.983.1 defines an access network that utilizes optical fiber technology to: (i) convey point to multipoint downstream traffic from an apparatus such as an Optical Line Termination (OLT) to multiple network units such as Optical Network Units (ONUs) or Optical Network Terminations (ONTs) and (ii) to convey point to point upstream traffic from an ONU or ONT to the OLT. For convenience of explanation both ONT and ONU will be referred as ONU.
An OLT may transmit information destined to multiple ONUs (said transmission is referred broadcast or multicast) or to transmit information destined to a single OLT (said transmission referred to as narrowcast). When the OLT receives information destined to multiple ONUs it multiplexes said information and transmits the multiplexed information to the ONUs. Each ONU is capable of detecting the information that is destined to it and to further process it while ignoring information destined to other ONUs.
An ONU provides, either directly or remotely, the user-side interface of the Optical Access Network (OAN). An OLT is provides the network side interface of the OAN and is connected, either directly or via an ODN, to multiple ONUs.
In a typical optical access network, such as but not limited to Broadband Passive Optical Network (BPON), EPON, G.983 compliant networks and G.984 compliant networks, the OLT as well as the multiple ONT or ONUs share the same upstream bit-rate as well as the same downstream bit-rate. For example, a G.983 compliant network defines an upstream bit-rate of 155 Mpbs and a downstream bit-rate of 622 Mbps. One previous standard defines both upstream and downstream bit-rate of 155 Mpbs.
An OLT controls upstream transmission from the ONUs by determining which ONU shall transmit by applying time division multiplex access (TMDA) control schemes. It usually operates on a fixed duration time-slot basis, and determines which ONT shall transmit upstream information at each time-slot. The ONUs are notified in advance when they allowed to transmit upstream information by means of Grants that are transmitted by the OLT. The OLT may apply various MAC schemes which are known in the art and do not require further explanation.
As each PON is adapted to operate at a certain fixed bit-rate, the OLT assigns bandwidth to ONUs by determining the amount of time-slots to allocate for each of said ONUs.
According to the ITU-T Recommendation G.983.4 upstream bandwidth may assigned in two manners—(a) in response to the utilization of upstream bandwidth by each of the ONUs, and (b) in response to upstream status reports from the ONUs or ONTs. The status report are transmitted upstream in mini-slots assigned by the OLT.
A prior art OLT is capable of allocating time-slots in response to multiple quality of service (QoS) levels. Typically, each ONU has at least one Transmission Container (T-CONT), each T-CONT has at least one queue.
An ONU reports the queue length of T-CONTs that belong to him. A typical T-CONT has one or more queues that are associated with one or more classes of service. Accordingly, the aggregate queues length of that T-CONT is reported.
Upstream and Downstream Data Structures
A typical optical access network supports Asynchronous Transfer Mode (ATM) based data transmission. Each ATM cell is 53 bytes long and includes a 5-byte header and 48-byte payload. The ATM header is utilized for routing ATM cells across the ATM based network.
Upstream traffic is arranged in an upstream frame of 53 time-slots. Each time-slot consists of three-bytes of PON layer overhead and either an ATM cell or a PLOAM cell.
The OLT allocates upstream bandwidth on a time-slot basis in response to the TCONTs queue length and then transmits downstream data grants in downstream PLOAM cells. Assuming that the upstream and downstream bit-rate are the same, then during a frame of 53 time-slots, two PLOAM cells are utilized for providing 53 data grants, corresponding to the 53 time-slots within each upstream frame. When the upstream data rate is much smaller than the downstream data rate, some PLOAM cells are empty.
In a typical PON downstream data is transmitted in serial data frames including multiple downstream slots, including framing slots and multiple ATM cell slots. Each downstream frame slot includes a field for transmitting upstream transmission permits. Upstream data is transmitted from an individual network unit in multiple upstream data slots, each upstream slot having a preamble portion and a payload portion.
Various schemes for upstream and down stream transmission over optical networks are illustrated at the following U.S. patent applications which are incorporated herein by reference: U.S. patent application serial number 20030219031 titled “Time-slot management method, a related network terminator, a related line terminator and an upstream signal frame structure” of Gyselings, et al.; U.S. patent application serial number 20030016692, titled “Method and system for processing upstream packets of an optical network” of Thomas, et al.; U.S. patent application serial number 20020030875 titled “method for requesting grant for MAC protocol in PON”, of Kim et al.; and U.S. patent application serial number 20030123483 titled “Method for dynamically allocating bandwidth in ATM passive optical network” of Kim et al.
Fixed Bit-Rate Receivers
The OLT has a downstream transmitter, for transmitting downstream information, and an upstream receiver, for receiving upstream information. Each ONU has an upstream transmitter, for transmitting upstream information, and a downstream receiver, for receiving downstream information. The exchange of information over the optical passive network takes into account the delays resulting from the distances between the OLT and each ONU. These delays are usually measured during an initialization stage that is also known as a ranging stage.
As mentioned above the upstream transmissions occur during time-slots. In many cases the downstream transmission forms a continuous time stream that eases the downstream synchronization process. The upstream receiver of the OLT is required to receive upstream information bursts from the multiple network units. Said reception usually includes various stages such as burst recognition, timing adjustments, clock and data recovery and the like.
Typically, the upstream receiver has an analog front end that is connected to a clock and data recovery (CDR) unit, a delimiter search unit, a byte align unit and a cell delineation unit. The upstream receiver may also include de-scrambling units, parsing units and the like.
The upstream receivers, and especially the CDR units and the cell delimitation units, are adapted to operate at a certain bit-rate, for example 622.08 Mpbs. Accordingly, an upstream receiver is designed to receive upstream information of a single bit-rate.
This single upstream bit-rate imposes various limitations upon the optical access network performance and may prevents network upgrades by adding high upstream rate ONUs and/or OLTs to the network. In order to overcome said bit-rate limitations ONUs having different upstream bit-rate can be added but this requires costly and complex adjustments such as allocating a new carrier wavelength for upstream transmission from said new ONU, adding additional connectivity between said ONU and the OLT, and the like.
There is a need to provide an apparatus and method for allowing upgrades of optical access networks as well as enhancing the flexibility of said access optical networks.